The field of the invention is superconducting motors and specifically squirrel cage motors that use superconductors. Superconductors are used for the purpose of increasing the power rating per unit weight of the motor and of reducing the electric Ohmic losses in the motor windings. Both of the latter improvements contribute to increasing the commercial value of the motor.
The early generation of superconducting rotating machines was restricted mostly to synchronous motors and alternators. Both of these machines require for their operation a fixed magnetic field generated by circulating a direct current in coil windings in the rotor which is connected to an external current source. The advantage of using superconductors in the rotor coil is to produce very high magnetic fields, considerably higher than can be generated by coil windings set in magnetic laminations. As a result, the synchronous motors and alternators are more compact and extremely efficient.
Synchronous electric motors employ a fixed magnetic field generated by the rotor acted upon by a rotating magnetic field created by the stator coils. The stationary magnetic field on the rotor constantly tries to align itself with the rotating magnetic field produced by the stator, causing the rotor to rotate and produce mechanical torque. Unfortunately, synchronous motors cannot operate at variable speeds and they are not self-starting. To obviate the two aforementioned drawbacks of the synchronous motor, direct current motors with superconducting field windings in the stator and conventional rotor coils with commutators were constructed and operated successfully.
The reason superconducting synchronous and direct current motors were the only superconducting electric motors to have been developed, so far, is due mainly to the limitations imposed by the properties of the commercially produced superconducting wires. There are two types of superconducting wires: the low temperature superconducting wires (LTS) and the high temperature superconducting wires (HTS). The low temperature hard superconductors, made of alloys of metals such as Nb and Ti, must be cooled down to temperatures below 10 degrees Kelvin for the material to be in the superconducting state. To ensure the stable operation of the LTS superconductor, the wire is made of many fine Nb-Ti strands, six microns in diameter, and embedded in a tube of copper. The other class of superconductors, made of alloys of earth metals (such as the so-called YBCO compounds) exhibit superconductivity at temperatures below 90 degrees Kelvin. The material, however, is very brittle. In order to shape it in the form of a long wire, it is encased in a silver tube and the whole matrix is extruded.
When an alternating current is caused to circulate in either class of wires (LTS or HTS) eddy currents are induced in the body of copper or silver. These secondary eddy currents lead to excessive Ohmic losses which will heat the superconductors and cause them quench (i.e. lose their superconducting property). This parasitic heating effect has discouraged the use of commercially available superconducting wires in motors excited with alternating current.
Because of recent advances in power electronics, alternating current drive systems have become a viable alternative to direct current motors for variable speed applications. Therefore there is a great deal of interest in searching for new methods of constructing superconducting alternating current motors that would incorporate two desirable features: the ability to exploit the attractive properties of available electronic power drives presently used with conventional alternating current motors and the use of HTS conductors. The HTS conductors require low cost liquid Nitrogen coolant and much simpler and more economical refrigeration systems that do LTS conductors.
The physics of the superconducting state are such that a superconducting ring behaves as a perfect diamagnetic body. Consequently, a current cannot be induced in a closed loop of superconducting material by using a moving magnetic field because the magnetic field cannot penetrate the superconducting loop. Conversely, if a magnetic flux had already penetrated a superconducting ring, the magnetic flux is frozen-in and cannot be destroyed.
The freezing of magnetic flux in a closed loop has been exploited by several inventors in different applications. Rabinowitz (U.S. Pat. No. 5,325,002) proposed to induce a current in a circuit of superconducting material that is above the critical temperature and therefore in the normal state. Once the current is induced, the circuit is then cooled to below its critical temperature at which time the material switches to the superconducting state and the circuit becomes a permanent electromagnet. A serious drawback of the Rabinowitz concept, if applied to the rotating field coils of a synchronous motor, is the significant amount of time it takes to cool the entire superconducting coil and its support. The long time constant renders the control of the motor completely impractical.
An extension of the Rabinowitz concept was applied by Leonard (1969), by Brechna and Kronig (1978), by Lipo (1987), by DeDonker and Novotny (1987) and by Tubbs (1990) to the construction of a synchronous/induction motor. This is a motor that is started as an induction motor and subsequently, when the rotor approaches the synchronous speed, the motor mode of operation is converted to that of a synchronous motor. In all of the above-mentioned motors, two sets of coils are installed on the rotor. The conductors of the first set are placed deep in the slots of the rotor laminations. The set of coils, made of superconducting wire, are inserted in the same slots and on top of the first set. When the motor is started all coils are at a temperature above critical. Because the Ohmic resistance of the superconducting coil at a temperature above critical is much higher than that of the non-superconducting first coil, upon starting the first coil carries most of the current and is responsible for the production of torque. Once the motor reaches the synchronous speed, the rotor is cooled to below critical temperature. As the second, superconducting coil is cooled, its resistance continually drops and a greater share of the current is transferred to it. Upon reaching superconductivity, the magnetic flux linking the coil becomes trapped in the coil and the motor behaves now as a synchronous motor.
All models of superconducting synchronous/induction motors constructed so far have displayed a disappointing performance because of two drawbacks. The first is due to the losses in the conductors. These consist of eddy current losses in the copper (or silver) matrix in the superconductor. The other drawback is the very long cooling time of the superconducting coil, which makes it impossible to alter the current in the coil while the motor is running in the synchronous mode.
It is possible, however, to induce a current in a circuit that is already below its critical temperature by creating a gap in the circuit to allow the magnetic field to penetrate the circuit. Once a current is produced in the circuit, the gap is closed and the current will continue to circulate indefinitely as long as the circuit remains superconducting. Furthermore, the Ohmic losses in the superconductor can be reduced or eliminated if the thermal stability of the conductors can be improved by proper shaping of the superconductors.
The present invention solves the two main problems that arise from the use of superconductors in the rotor of a synchronous electric motor. First, rather than incurring the costs and complexity involved in forming HTS coils, this motor uses superconducting material deposited in a thin film on the outer surface of a rotor of insulating material. The use of superconductors in the form of thin films removes the need for sheaths of copper of silver to provide mechanical integrity and added heat capacity. With the absence of the sheath, eddy currents are no longer generated in the conductors and with it the Ohmic losses that are the one serious drawback to the use of superconductors with alternating current.
Second, this motor induces a current in the superconducting material after it has been cooled to below its superconducting temperature by creating a gap in the circuit, although it creates this gap without using a mechanical switch. Instead, this motor raises the temperature of a small portion of the superconducting material so that is no longer superconducting. This heated portion then becomes the xe2x80x9cgapxe2x80x9d in the circuit that allows a current to be induced. The heated portion has a heat capacity that is a very small fraction of the total heat capacity of the rotor. As a result, the xe2x80x9copeningxe2x80x9d and xe2x80x9cclosingxe2x80x9d of the gap takes place in milliseconds, an interval of time more than sufficient for regulating the current in the rotor.
Specifically, the present invention provides a conventional stator winding in a configuration well known in the art. Located inside the stator is a cylindrical rotor whose outer surface is covered by a thin film of superconducting material. Imbedded within the rotor or otherwise connected to it is an auxiliary squirrel cage composed of a non-superconducting material or other motor that is used to provide startup torque to bring the rotor to a synchronous speed. The present invention also contains a provision to cool the superconducting material to a critical temperature below which it becomes superconducting, and a heating device to raise a portion of the superconducting material to a temperature above its critical temperature once the rotor is at the synchronous speed.
It is thus one object of the invention to fix a magnetic field in a rotor that is already largely precooled to superconducting temperatures and to generate a magnetic field that rotates around the axis of the rotor by exciting the stator winding with an AC current. The formation of a rotating magnetic field in a stator winding is well known in the art and is a component of a conventional non-superconducting synchronous motor.
The thin film of superconducting material does not cover the entire outer surface, but instead is deposited in a pattern resembling a traditional squirrel cage rotor, with bars of superconducting material parallel to the axis of rotation that are joined at both ends by rings of superconducting material around the circumference of the rotor cylinder. The surface configuration of the superconducting material in the shape of a squirrel cage allows the current to circulate in loops around the squirrel cage, thus creating a fixed magnetic field whose number of poles is dictated by the number of bars in the surface squirrel cage. It is thus an object of the invention to eliminate the need to form superconducting material into thin wires. This would save considerable cost and complexity in the manufacture of superconducting motors.
Surrounding the central shaft are several layers of iron laminations. Imbedded in these laminations is a conventional squirrel cage rotor coil composed of a non-superconducting material. The purpose of this conventional squirrel cage is to produce start-up torque as described below. It is thus one object of the invention to make as compact a motor as possible by imbedding the auxiliary squirrel cage in the rotor.
Firmly bonded to the iron laminations is a cylindrical ceramic shell. This shell provides a base for the thin film of superconducting material. The superconducting film is not deposited directly on the ceramic shell, however. Instead, a substrate is applied in a thin layer on the surface of the ceramic shell. The purpose of the substrate is to provide a surface structure that is compatible with the superconducting material. In the preferred embodiment of the invention, this substrate consists of compounds such as zirconia (ZrO2).
In the preferred embodiment of the present invention, the device for heating the superconducting material is a laser that can be pulsed on to provide a short burst of energy. This laser can be located within the stator winding or it can be located external to the stator winding with a length of fiber-optic cable providing a means to focus the laser beam on the rotor. It is thus one object of the invention to provide a simple means for heating a small portion of a rotating rotor that can be switched synchronously with the AC drive.
The operation of this motor was designed to reduce the amount of complexity normally associated with superconducting motors. First, the rotor is cooled so that the thin film becomes superconducting. Then, a rotating magnetic field is generated in the stator coils by exciting them with AC currents. This rotating magnetic field cannot xe2x80x9clinkxe2x80x9d with the superconducting material because of the diamagnetism of the superconducting material. However, this rotating magnetic field induces a current in the auxiliary squirrel cage that is composed of normal conducting material and imbedded within the iron laminations of the torque tube. This induced current generates its own magnetic field that interacts with the stator field to produce rotation in the rotor. At this point, the motor is acting as a traditional squirrel cage induction motor. As the rotor approaches synchronous speed, a portion of one of the xe2x80x9cbarsxe2x80x9d of the superconducting squirrel cage is heated so that it is no longer superconducting. The formation of this resistive spot on the squirrel cage creates a gap in the superconducting circuit and allows the magnetic field to penetrate the circuit. The heated spot is then allowed to cool back down and the current is then trapped in the superconducting squirrel cage. This trapped current creates a permanent magnetic field in the rotor and the motor then operates as a synchronous motor.
The foregoing and other objects and advantages of the invention will appear from the following description. In the description, reference is made to the accompanying drawings that form a part hereof, and in which there is shown by way of illustration, a preferred embodiment of the invention. Such embodiment does not necessarily represent the full scope of the invention, however, and reference must be made to the claims herein for interpreting the scope of the invention.